This protocol will study infants with birth weights between 501 and 1500 grams. Data are routinely collected using standardized forms and definitions. Data are abstracted by the research nurses from the mothers' and infants' charts, entered into a computer that applies specific error checks and then transmitted via telephone modem to the George Washington Biostatistical Coordinating Center. In instances where infants are transferred to another facility, final disposition is tracked until the infant dies or is discharged to home.